target/riscv: Simplify riscv_cpu_gdb_write_register()

Use a single ldn() call, sign-extend once.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260318103122.97244-9-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>